experimental characterisation of a downdraft gasifier fueled with woodchar/woodchips mixtures

1° Convegno SIBA Salerno, 22-23/10/07

F. Pittaluga – University of GenoaDIMSET/SCL – Savona Combustion [email protected] - http://proxy.sv.inge.unige.it/SCL/

SOCIETA’ ITALIANA BIOENERGIA E AGROINDUSTRIA

Experimental Characterisation of a Downdraft Gasifier Fueled with Woodchar/Woodchips Mixtures




Biomass Gasification: Fundamentals

Biomass gasification is basically a conversion of solid fuels (wood, wood-waste, agricultural residues, charcoal, etc.) into a combustible gas mixture normally called Producer Gas.

The process is typically used for various biomass materials and it involves partial combustion of such biomass.

Partial combustion process occurs when air supply (O2) is less than adequate for the combustion of biomass to be completed.





Biomass is usually composed of the elements carbon, hydrogen and oxygen. In addition there may be nitrogen and sulphur, but since these are present only in small quantities they will be disregarded in the following discussion.

In the types of gasifiers considered here, the solid fuel is heated by combustion of a part of the fuel. The combustion gases are then reduced by being passed through a bed of fuel at high temperature.

In complete combustion, carbon dioxide is obtained from the carbon and water from the hydrogen. Oxygen from the fuel will of course be incorporated in the combustion products, thereby decreasing the amount of combustion air needed.





Oxidation of carbon, or “combustion”, is described by the following chemical reaction formulae (auto thermal gasification): C + O2 CO2 - 393.8 kJ/mol (exo-thermic) C + ½ O2 CO - 123.1 kJ/mol (exo-thermic)

In all types of gasifiers, CO2 and H2O (vap.) are converted (reduced) as much as possible to CO, H2 and CH4 , which are the main combustible components of producer gas.





The most important reactions that take place in the reduction zone of a gasifier between the different gaseous and solid reactants are the following

C + CO2 2 CO + 159.9 kJ/mol (Boudouard, endo-th.) C + H2O CO + H2 + 118.5 kJ/mol (water gas reaction, endo-th.) CO2 + H2 CO + H2O + 40.9 kJ/mol (water gas shift reaction, endo-th.) C + 2 H2 CH4 - 87.5 kJ/mol (methanisation, exo-thermic)





Despite the substoichiometric conditions prevailing everywhere, oxidation of CO and H2 may locally take place according to:

CO + ½ O2 CO2 - 283.9 kJ/mol (exo-thermic) H2 + ½ O2 H2O - 285.9 kJ/mol (exo-thermic)

Though they produce heat, to the advantage of the auto-thermal gasification, they are undesired because the producer gas heat-value is lowered.





The equilibrium of above seen “water gas” (heterogeneous) and “water gas shift” (homogeneous) reactions largely govern the final gas composition.

On the other hand, the equilibrium composition of the gas will only be reached when the reaction rate and the time for reaction are adequate.

Below 700 °C the reaction rates proceed so slowly that the product gas composition is “frozen” : once formed, the gaseous products do not further react with each other. Unfortunately, this applies also to the tars, which build up in the gas, since they cannot undergo any “reduction”.




Biomass Gasification at DIMSET/SCL

Manufacturer:

ANKUR Ltd.

located near the ancient “Sama Jakat Naka”

Baroda-390008, Gujarat, India

web site: www.ankurscientific.com

http://www.ankurscientific.com/

http://www.ankurscientific.com/





To a Gas EngineOr Gas Turbine

Conceptual Sequence of Processes from Biomass to End Products

Product GasCleaning Equipment




Biomass Gasification at DIMSET/SCL Fuel Parameters, Gasifier Performance, Product Gas, Slag, Emissions Monitoring - Physico-chemical characterisation of fuel (woodchips and charcoal-woodchips mix) - Air/Fuel Equivalence Ratio●- Inner temperature profile (overall gasifier) - Pressure drop (@ sand filter)●- Product gas temperature (@ gasifier outlet) - Water temperature (@ scrubbers)●- Product gas flow rate (@ gasifier outlet) - Water pressure (@ scrubbers) ●- Inner gasifier pressure - Water flow rate (@ scrubbers) ●- “Clean” product gas chemical composition - Condensate chemical composition ●- Slag composition and mass flow rate - Condensate mass flow rate

●- Exhaust gas composition and mass flow rate (@ gas engine outlet)●- Co-generation thermal power (@ overall equipment and gas engine)





HEARTH LOAD OPTIMISED FOR MINIMUM TAR CONTENT

Temperature Distributions for Varying Fuel Typologies

Wood@32% hu

Wood@25% hu

W@15% Coal

W@30% Coal

S-2

S-3

S-4

TC3

TC2

TC1

TC4





HEARTH LOAD OPTIMISED FOR MAX GAS ENERGY

Temperature Distributions for Varying Fuel Typologies

Wood@32% hu

Wood@25% hu

W@15% Coal

W@30% Coal

S-1

S-2

S-3

S-4

TC3

TC2

TC1

TC4

S-1

Wood@32% hu

S-2

S-3

S-4

Wood@25% hu

Wood@15% Coal

Wood@30% Coal

TC3

TC2

TC1

TC4




Product Gas Heat Values for Varying Fuel Typologies

TC1TC1

S-2

S-3

S-4

Wood@25% hu

Wood@15%Coal

Wood@30%Coal

Product Gas Heat Values

0

500

1000

1500

2000

0 0.1 0.2 0.3 0.4 0.5 0.6

Superficial Velocity (m/s)

Hea

t Val

ue H

i (kC

al/N

m3)

Serie1

Serie2

Serie3

S-2

S-2

S-2

Wood@25% hu

Wood@15% Coal

Wood@30% Coal




Product Gas Flow Rate for Varying Fuel Typologies

TC1TC1

Product Gas Flow Rate

0

10

20

30

40

50

60

0 0.1 0.2 0.3 0.4 0.5 0.6


Syng

as F

low

Rat

e (N

m3/

s)

Serie1

Serie2

Serie3

S-2

S-3

S-4

Wood@25% hu

Wood@15% Coal

Wood@30%Coal

S-2




Gas Thermal Power for Varying Fuel Typologies

TC1TC1

Gas Energy Flow Rate (kCal/h)

0

10000

20000

30000

40000

50000

0 0.1 0.2 0.3 0.4 0.5 0.6


Gas

Ene

rgy

(kC

al/h

)

Serie1

Serie2

Serie3

S-2

S-3

S-4

Wood@25% hu

Wood@15% Coal

Wood@30% Coal




Electric Power (kW)

024

68

1012

1416

0 0.1 0.2 0.3 0.4 0.5 0.6


Elec

tric

Pow

er (k

W)

Serie1

Serie2

Serie3

Electric Power Generation for Varying Fuel Typologies

S-2

S-3

S-4

Wood@25% hu

Wood@15% Coal

Wood@30% Coal




Tar + Particulate in Gas

01000200030004000

5000600070008000

0 0.1 0.2 0.3 0.4 0.5 0.6


Tar +

Par

ticul

ate

(mg/

m3)

Serie1

Serie2

Serie3

Tar and Particulate Levels for Varying Fuel Typologies

S-2

S-3

S-4

Wood@25% hu

Wood@15% Coal

Wood@30% Coal




Tar + Particulate in Gas

050

100150200250300350400

0 0.1 0.2 0.3 0.4 0.5 0.6


Tar +

Par

ticul

ate

(mg/

m3)

Serie1

Serie2

Serie3

Tar and Particulate Levels for Varying Fuel Typologies

S-2

S-3

S-4

Wood@25% hu

Wood@15% Coal

Wood@30% Coal

IN ORDER TO QUANTIFY THE LOWER CONCENTRATIONS OF TARS,THE VALUES REPORTED ARE LIMITED TO SV HIGHER THAN 0.25 M/S




Conclusions

The experimental results above presented can be considered as safely demonstrative of the actual possibility, provided by the fixed-bed downdraft gasification technology, of successfully pursuing a widespread territorial distribution of micro CHP (combined heat and power) generating plants fueled by wood/charcoal mixtures.

One main outcome of the investigation has been that of unequivocally assessing the great potential, for a given gasifier technology, residing in its functional optimisation, to be achieved by imposing correct operative parameters, most important among all, the hearth load.




Conclusions

Without the need of re-designing or complicating an intrinsically simple and reliable basic technology, the performance parameters such as:

• power plant efficiency• product gas yield• and, most importantly, tar and particulate content in the gas

can be optimised by means of a careful functional-variables setting.




ConclusionsThe outcome of pursuing a suitable mixing of woodchips with charcoal has turned out extremely beneficial under all respects:

• gas yield and gas energy flow, increasing almost proportionally with the increase of charcoal percentage• decrease of tar content in the product gas, thanks to the higher hearth temperatures• “thermal nobilitation” of lower heat content woodchips, possibly even of green waste too

From this perspective, a renewed interest in charcoal production, with up-to-date technologies (e.g. auto-thermal retorts) could be conceived, with interesting environmental and economic returns.

experimental characterisation of a downdraft gasifier fueled with woodchar/woodchips mixtures

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